The present invention relates to the process control industry. More specifically, the present invention relates to field devices used in the process control industry which communicate using two-wire process control loops.
Field devices such as transmitters, are used in the process control industry to remotely sense a process variable. The process variable may be transmitted to a control room for use in controlling the process or for providing information about process operation to a controller. For example, information related to pressure of process fluid may be transmitted to a control room and used to control the process, such as oil refining. Another type of field device, for example, is a valve controller.
One typical prior art technique for transmitting information involves controlling the amount of current flowing through a process control loop. Current is supplied from a current source in the control room and the transmitter controls the current from its location in the field. For example, a 4 mA signal can be used to indicate a zero reading and a 20 mA signal can be used to indicate a full scale reading. More recently, transmitters have employed digital circuitry which communicates with the control room using a digital signal which is superimposed onto the analog current signal flowing through the process control loop. Some techniques send purely digital signals. One example of such a technique is the HART® communication protocol proposed by Rosemount Inc. The HART® protocol and other such protocols typically include a set of commands or instructions which can be sent to the transmitter to elicit a desired response, such as transmitter control or interrogation.
Fieldbus is a communications protocol proposed by the Fieldbus Foundation and is directed to defining a communications layer or protocol for transmitting information on a process control loop. In the Fieldbus protocol, the current flowing through the loop is not used to transmit an analog signal. Instead, all information is digitally transmitted. Further, the Fieldbus standard allows transmitters to be configured in a multi-drop configuration in which more than one transmitter is connected on the same process control loop.
The Fieldbus standard is a specification promulgated by the Fieldbus foundation. The process interface specification is defined in “The Fieldbus Foundation, Fieldbus Specification, Function Block Application Process Parts 1 and 2”, Documents FF-94-890 and FF-94-891, Revision H1 Final 2.0, Jan. 2, 1996. The Fieldbus standard is an open standard which provides communication between process I/O hardware and the Fieldbus interface through an arbitrary number of data channels, each having an arbitrary number of parameters associated therewith. A Fieldbus process interface specification (transducer block) appears as shown in Table 1:
TABLE 1Channel DataParameter 1Parameter 2Parameter 3...Parameter NFieldbus interface circuitry in the transmitter reads and writes the channel data and parameters 1-N through software constants termed “function blocks” in the Fieldbus Foundation Specification.